1. Field of the Invention
The invention is in the field of imaging devices and more particularly in the field acoustic lenses for ultrasonic imaging.
2. Description of Prior Art
Ultrasonic imaging is a frequently used method of analysis for examining a wide range of materials. Ultrasonic imaging is especially common in medicine because of its relatively non-invasive nature, low cost, and fast response times. Typically, ultrasonic imaging is accomplished by generating and directing ultrasonic sound waves into a medium of interest using a set of ultrasound generating transducers and then observing reflections generated at the boundaries of dissimilar materials, such as tissues within a patient, also using a set of ultrasound receiving transducers. The receiving and generating transducers may be arranged in arrays and are typically different sets of transducers, but may differ only in the circuitry to which they are connected. The reflections are converted to electrical signals by the receiving transducers and then processed, using techniques known in the art, to determine the locations of echo sources. The resulting data is displayed using a display device, such as a monitor.
Typically, the ultrasonic signal transmitted into the medium of interest is generated by applying continuous or pulsed electronic signals to an ultrasound generating transducer. The transmitted ultrasound is most commonly in the range of 40 kHz to 15 MHz. The ultrasound propagates through the medium of interest and reflects off interfaces, such as boundaries, between adjacent tissue layers. Scattering of the ultrasonic signal is the deflection of the ultrasonic signal in random directions. Attenuation of the ultrasonic signal is the loss of ultrasonic signal as the signal travels. Reflection of the ultrasonic signal is the bouncing off of the ultrasonic signal from an object and changing its direction of travel. Transmission of the ultrasonic signal is the passing of the ultrasonic signal through a medium. As it travels, the ultrasonic energy is scattered, attenuated, reflected, and/or transmitted. The portion of the reflected signals that return to the transducers are detected as echoes. The detecting transducers convert the echo signals to electronic signals and, after amplification and digitization, furnishes these signals to a beam former. The beam former in turn calculates locations of echo sources, and typically includes simple filters and signal averagers. After beam forming, the calculated positional information is used to generate two-dimensional data that can be presented as an image.
As an ultrasonic signal propagates through a medium of interest, additional harmonic frequency components are generated. These components are analyzed and associated with the visualization of boundaries, or image contrast agents designed to reradiate ultrasound at specific harmonic frequencies. Unwanted reflections within the ultrasound device can cause noise and the appearance of artifacts (i.e., artifacts are image features that result from the imaging system and not from the medium of interest) in the image. Artifacts may obscure the underlying image of the medium of interest.
One-dimensional acoustic arrays have a depth of focus that is usually determined by a nonadjustable passive acoustic focusing means affixed to each transducer. This type of focusing necessitates using multiple transducers for different applications with different depths of focus.
The width of the beam determines the smallest feature size or distance between observable features that can be observed. The imaging system determines position by treating the beam as if it had essentially a point width. Consequently, efforts have been made to achieve a narrow beam of focus, because when the beam is wide, features that are slightly displaced from the point of interest also appear to be at the point of interest. The longer the region having a narrow beam of focus, the greater the range of depth into the medium of interest that can be imaged.
The beam intensity as a function of position may oscillate rather than fall off monotonically as a function of distance from the center of the beam. These oscillations in beam intensity are often called xe2x80x9cside lobes.xe2x80x9d In the prior art, the term xe2x80x9capodisationxe2x80x9d refers to the process of affecting the distribution of beam intensity to reduce side lobes. However, in the remainder of this specification the term xe2x80x9capodisationxe2x80x9d is used to refer to tailoring the distribution of beam intensity for a desired beam characteristic such as having a Guassian or sinc function (without the side lobes) distribution of beam intensity.
Steering refers to changing the direction of a beam. Aperture refers to the size of the transducer or group of transducers being used to transmit or receive an acoustic beam.
The prior art process of producing, receiving, and analyzing an ultrasonic beam is called beam forming. The production of ultrasonic beams optionally includes apodisation, steering, focusing, and aperture control. Using a prior art data analysis technique each ultrasonic beam is used to generate a one dimensional set of echolocation data. In a typical implementation, a plurality of ultrasonic beams are used to scan a multi-dimensional volume.
FIG. 1A shows a prior art acoustic focusing system 100A, having a lens 102A with a simple (i.e., a non-compound) surface, focusing a beam 104A, into a focused region 106, having a depth of focus 108. FIG. 1A is a two dimensional depiction of the acoustic art focusing system 100A. The third dimension is not discussed in conjunction with FIG. 1A, but will be discussed in conjunction with FIGS. 1B and 1C. In contrast to the usage of the terms xe2x80x9csimplexe2x80x9d and xe2x80x9ccompoundxe2x80x9d in optics, in the context of this specification simple and compound are used to describe the complexity of the curvature of the lens surface. Similarly, in this specification a lens having a compound surface curvature may be referred to as having a compound surface. If for each side of the lens the curvature can be described as one mathematically smooth and continuous curve of the same concavity or convexity, the lens is simple even if each side of the lens is characterized by a different curve. Otherwise, the lens and its associated curvature are complex or compound.
Lens 102A is an acoustic lens, and beam 104A is an ultrasound beam. The distance from lens 102A to the center of focused region 106 is the depth of focus 108. The focused region 106 represents a range of focus in which the beam is in focus. As long as the velocity in the medium surrounding lens 102A is greater than in lens 102A, a convex curvature will tend to focus beam 104A to a point. When the velocity in the medium surrounding lens 102A is lower than in lens 102A a concave curvature will focus beam 104A to a point or line.
The depth of focus 108 in ultrasonic imaging may be a significant parameter in obtaining high resolution. The direction of the depth of focus is normally taken to be perpendicular to the direction along which phased elements are aligned (in the downstream direction).
The prior art utilizes an acoustic lens, such as lens 102A, of a fixed focus and relies upon a typical depth of focus of the acoustic beam, such as beam 104A, during penetration of the signal into a medium of interest. The range of the focus or the length of the focused region 106 is often inadequate for imaging many of the different organs or regions of the human body, for example, that may constitute the medium of interest. One reason the range of focus may be inadequate is because the size of the medium of interest such as an organ may be larger than the focused region. Consequently, for some mediums of interest it may be necessary to switch lenses and/or transducer lenses to image the entire medium of interest when using a lens such as lens 102A. Efforts have been made to extend the length of the focused region 106 by using lenses with compound surfaces.
FIG. 1B shows a prior art acoustic focusing system 100B having a spherical lens 102B, and a beam 104B. The beam 104B becomes a line as it comes to its focus and therefore has a cross section perpendicular to its direction of propagation that is a circle or is ideally a point.
FIG. 1C shows a prior art acoustic focusing system 100C having a cylindrical lens 102C, and a beam 104C. The beam 104C becomes a sheet as it comes to its focus and therefore has a cross section perpendicular to its direction of propagation that is a rectangle or is ideally a line.
Acoustic focusing systems 100B and 100C are examples of acoustic focusing system 100A.
FIGS. 1D-F show ultrasound transducer arrays and aid in understanding terminology used in the ultrasound art. FIGS. 1D-F have transducer arrays 118D-F, transducer elements 120D-F, and coordinate system 122. Coordinate system 122D defines the elevation direction along its vertical axis and the azimuthal direction along its horizontal axis. In the ultrasound art the term one-dimensional or 1D array (e.g., transducer array 118D) refers to an array of transducers (e.g., transducer elements 120D) that consists of a single row of transducer 120D. Often each transducer in the row has a length in elevation direction that is significantly longer than its width in the azimuthal direction. The 1D array allows for steering in only the azimuth direction. The term two dimensional or 2D array (e.g., transducer array 118F) refers to an essentially square array of transducers including nearly the same number of rows as columns, in which the individual transducer elements can be square or rectangular, for example. In contrast to the 1D array, the 2D array allows for beam steerng in any direction, which is useful in 3-D imaging. Similarly the term 1.5D (e.g., transducer array 118E) refers to an array of transducers, which contains more than one row of transducers (e.g., transducer elements 120E) in the azimuthal direction. The 1.5D array may use phasing, for example in the elevation direction form improved beam characteristics. The terms 1.75D and 1.8D and similar terms greater than 1.5D are used to refer to arrays that have a number of rows in the azimuthal direction that is between that of the 1.5D and the 2D arrays.
FIG. 2 shows a prior art focusing system 200 having a lens 202 with a compound surface. This lens 202 includes an inner lens portion 204 and outer lens portion 206 joined at a ring that forms cusp 207. Beam 208 has an inner beam portion 210 and outer beam portion 212 that travels predominantly through inner lens portion 204 and outer lens portion 206, respectively. FIG. 2 also includes near focused region 214, far focused region 216, and coordinate system 218.
The use of different portions of lens 202 with different radii of curvature, or different degrees of concavity or convexity, results in different focal points. Upon exiting lens 202, inner beam portion 210 is focused into near focused region 214, whereas outer beam portion 212 is focused into far focused region 216. The near focused region 214 and far focused region 216 combined form a range of focus that may be greater than is possible for lens 102A and is greater than either the near focused region 214 or the far focused region 216 alone. In one embodiment inner beam portion 210 and outer beam portion 212 are separate beams applied at different times. When using the near focused region 214 the focusing system 200 is said to be operating in near penetration. When using the far focused region 216 the focusing system 200 is said to be operating in far penetration. Alternatively, inner beam portion 210 and outer beam portion 212 may be the same beam or travel during overlapping time periods. Coordinate system 218 is used to characterize the shape of lens 202 as a curve, z, that is a function of a radial direction r and an angular direction xcex8, or z(r,xcex8), that describes the shape of the downstream side of lens 202. A circular convex or concave lens, such as lens 102A is symmetrical about the z axis and therefore z(r,xcex8) is independent of angle xcex8 and consequently can be written as z(r). The lens may be circular or cylindrical, having different regions of different curvature. At cusp 207 curve z(r) is mathematically continuous. However, at cusp 207 the first and second derivatives of the curve, zxe2x80x2(r) and zxe2x80x3(r), are not continuous, and are essentially undefined.
Although possibly not recognized in the prior art, different curvatures on the lens surface of lens 202 result in difficulties of acoustic contact with a medium of interest, such as a human body. These difficulties are highlighted when as a result of different curvatures, some of the coupling gel and/or air bubbles are trapped in different segments of the transducer surface or between the medium of interest and the compound surface of the lens. The coupling gel tends to distort the shape of compound lenses, such as lens 202, thereby distorting its focusing characteristics. Another problem recognized by the present inventors is that the increased thickness of the inner lens portion 204 has an increased attenuation of the signal causing poor signal return. This problem is exacerbated because the inner lens portion 204 is normally used for higher frequencies, which are particularly sensitive to attenuation by thicker lenses. The attenuation characteristics of lenses 102A and 202 result in an angular distribution of beam intensity that is low in the center and high at the edges, and is thereby nearly the inverse of a Guassian distribution. However, it is desirable to have a Guassian distribution of beam intensity to maintain a sharp focus.
An acoustic lens having a non-compound or simple curvature is provided in which different segments or regions of the lens have different acoustic indices of refraction. In many materials, greater amounts of heating, curing, or irradiating with various types of particles or radiation yield greater amounts of material crosslinking, which makes the material harder. In general, however, greater amounts of heating, curing, or irradiating changes the material in a variety of ways such as by increasing or decreasing the amount of crosslinking, the density, and/or hardness. Each region may include different materials, or the same material treated (e.g., cured, irradiated, or heated) differently. These variations in materials may be used to associate different compressibilities and/or different densities with different lens regions, thereby setting different indices of refraction to those regions, for example.
The different focal length portions of the acoustic lens may coincide with different portions of a transducer surface. The different portions of the transducer surface may have different transmit and receive frequency characteristics. A range of frequency can be referred to as a transducer frequency domain. Thus, the different portions of the transducer surface can be associated with different transducer frequency domains. Coupling the different transducer frequency domains with different focal length portions helps extend the focused region of the lens so that it has a sharp focus beyond what is feasible with the prior art.
Further, the transducer or transducer array may be shaped so that different frequencies excite different portions of the transducer or transducer array. The chosen frequency of operation may be higher for shallow penetration into a medium of interest such as a human body, for example. The high frequency portion of the transducer may be aligned with the lens portion having the more shallow focus or shorter focal length, and the low frequency portion of the transducer may be aligned with the portion of the lens having the deeper focus or longer focal length. In this way, the portion of the transducer and the lens associated with the longer focal length will be inactive. An inactive portion will not interfere with the lens"" focal quality when activating the portion of the transducer and lens associated with the shorter focal length, and visa versa. In addition to the velocity or compressibility and the density of the lens medium or material, the acoustic attenuation can also be tailored to optimize beam characteristics. For example, the sections of the lens intended to focus low frequency acoustic energy can have a higher attenuation factor than the sections intended to function at higher frequencies. Since attenuation increases at higher frequencies, the sections of the lens that will function at low frequencies will tend to filter out higher frequencies. This feature will allow the construction of devices that will approach the performance of 1.5D, 1.75D, or 1.8D transducers with simpler electronic switches, and can be used for shaping the intensity distribution of the beam or apodisation. Extending the focus will involve only disconnecting the central row or rows of the array when operating at low frequency in the far penetration mode. Connecting and disconnecting the central row or rows while the outer rows remain connected is easier than connecting and disconnecting both the inner and outer rows such that the inner and outer rows are not functional simultaneously.
Broad beam technologies refer to systems and methods that include or take advantage of techniques for generating ultrasound and analyzing detected echoes, broad beam technologies use multidimensional spatial information obtainable from a single ultrasonic pulse.
Area forming is the process of producing, receiving, and analyzing an ultrasonic beam, that optionally includes apodisation, steering, focusing, and aperture control, where a two dimensional set of echolocation data can be generated using only one ultrasonic beam. Nonetheless, more than one ultrasonic beam may still be used with the area forming even though only one is necessary. Area forming is a process separate and distinct from beam forming. Area forming may yield an area of information one transmit and/or receive cycle, in contrast to beam forming that typically only processes a line of information per transmit and/or receive cycle. Alternatively, beam forming can be used instead of area forming electronics throughout this application.
Volume forming is the process of producing, receiving, and analyzing an ultrasonic beam, that optionally includes apodisation, steering, focusing, and aperture control, where a three dimensional set of echolocation data can be generated using only one ultrasonic beam. Nonetheless, multiple ultrasonic beams may be used although not necessary. Volume forming is a superset of area forming.
Multidimensional forming is the process of producing, receiving, and analyzing an ultrasonic beam, that optionally includes apodisation, steering, focusing, and aperture control. Using multidimentional forming a two or more dimensional set of spatial echolocation data can be generated with only one ultrasonic beam. Nonetheless, multiple ultrasonic beams may be used although not necessary. Multidimensional forming optionally includes non-spatial dimensions such as time and velocity.
The present acoustic lens can be used with broad beam technologies, area forming, volume forming, or multidimentsional forming. Alternatively the present acoustic lens can also be used with beam forming. When used with area forming the acoustic lens is typically cylindrical so as to allow the use of a broad beam that has across section shaped like a line rather than a point and is focused along its height, but not along its width.